Frequency sensitive elements



Nov. 10, 1942. J. s. STONE 2,301,828q

FREQUENCY SENSITIVE ELEMENTS l Filed April 12, 1940 4 sheets-she@ I C /freuencg 5ans/fire, E/emen 1 Z 3 Al f ll f m f" AmgNEY Nov. 10, 1942. J. s. sToNE v v FREQUENCY SENSITIVE ELEMENTS 4 Sheets-Sheet Nov. 109 1942. J. s. STONE FREQUENCY SENSITIVE ELEMENTS Filed 'April l2, 1940 4 Sheets-Sheet 3 ATTORNEY Nov.y 10, 1942. J. s. STONE 2,301,828

FREQUENCY SENS ITIVE ELEMENTS resonant circuit.

Patented Nov. 10, 1942 sA PATENT OFFICE FREQUENCY SENSITIVE ELEMENTS l John Stone Stone, San Diego,'Calif., assigner to American Telephone and Telegraph a corporation of New York Company,

Application April 12, 1940, Serial No; `329,380

14. Claims. (CLIN-171.5)

An object of my invention is to produce improved frequency sensitive elements.

Another object of this invention is to produce an amplifying modulator. Another object of my invention is to produce a constant frequency oscillator.V v

These and various other objects of my invention will be made apparent in the following speciflcation and claims, taken with the accompanying drawings in which I have illustrated a limited number of specific embodiments of the invention.

The term frequency l sensitive elements" as used in this specification signifies a device which has an impedance-frequency curve whose slope at a point of inflection is very steep. Thus if Z be the impedance of the device at a point of inflection corresponding to the frequency n, and if no be the resonance or the antiresonance frequency of the device nearest to n, then 0l/m) has been taken as the measure of the steepness of the impedance curve at said point of inection.

In United States Patent No. 2,232,891 to J. S. estone, February 25, 1941, the simple antiresonant circuit of Figure was disclosed as the frequency sensitive element in an amplifying modulator and the mode of operation of the frequency sensitive element in the operation of modulating a high frequency current was there fully disclosed.

The present invention will be best understood by having reference to the accompanying drawings. Figures 1, 2, 3, 3a and 4 are highly schematic diagrams illustrative of some of the uses to which frequency sensitive elements may be put.

Figure 5 is a circuit diagram of a simple anti- Fig. 5a is a circuit diagram used to distinguish between the resonant and antiresonant circuit. Figs. 6a, 7a, 8a, 9a, 12a and 13a illustrate examples of frequency sensitive elements,- while Figs. 6b, 7b, 7c, 8b, 9b, 9c, 12b, 13b,` 13e illustrate, respectively, these frequency sensitive elements equipped with condenser telephone transmitters adapted to fluctuate the critical frequency of the element. Fig. 10 is a circuit diagram illustrating the circuit equivalent of a piezo electric crystal when vibrating to frequencies in the neighborhood of its resonance frequency n and of its antiresonance frequency n..

Fig. 11 is a circuit diagram used to explain the function and mode of operation of the crystal K' in the organizations of Figs. 12a, 12b, 13a and a condenser telephone transmitter may have its apparent capacity magnified. Fig. 15 illustrates how a crystal may have its antiresonance point lowered. Fig. 16 illustrates a feed-back arrangement between the 4modulator and constant frequency source of energy, and also an arrangement whereby, if desired, the carrier frequency may be suppressedin the output or utilization circuit 3, 3. Fig. 17 illustrates the orientation of a crystal section in a quartz crystal. I'he crystal section here shown is known as the Curie cut and is used as the specific crystal of reference in this specication. Fig. 18 is a set of curves illustrating more specifically the mode of operation of the organization of Figs. 6a and 6b. Fig. 19 is a set of curves illustrating more specifically the mode of operation of the organizationv of Figs. 8a and 8b. Fig. 20 is a set of curves illustrating one mode of operation of the devices of zo Figs. 7a and 7b, and the characteristic curves of one of our simpler crystals at and in the immediate vicinity of its resonant frequency. Fig. 21 is a curveillustrating the mode of operation of the device of Figs. 9a and 9b. Fig. 22 is a set of 25' curves illustrating the mode of operationy of'Figs.

In Figs. 1, 2, 3, 3a and 4 the device A is a source of constant and high frequency electromotive force, B is a frequency sensitive element, C (which does not occur in Fig. 3) is a` condenser telephone transmitter, and D is any suitable transforming or translating device adapted to pass the energy of the high frequency current on to the utilization circuit 3, 3', either as an amplitude modulated high frequency current or as an audio frequency current depending on the'nature of D. l, l', 2, 2', I, I', l, 4' are conductors.

In Fig. 1 the frequency elements are combined to form a modulator circuit in which the con- 40 denser transmitter C controls the frequency sen-1 sitive element B. Fig. 2 is a similar circuit in y which'thetransmitter C controls the source A. In Fig. 3 the source A and frequencysensitive element B are so connected that the frequency of the source A is governed by the natural period of the frequency sensitive element B. In Fig. 3a this combination is used to supply the carrier frequency to a second. frequency sensitive ele- 13b. Fig. 14 is a circuit diagram illustrating how 55 condenser transmitter C.

i frequency of the crystal is 250,000, each such interval comprises cycles.

Curve 3 discloses for the rst time, the useful though theoretically unimportant fact that the maximum value of the effective positive reactance P of the crystal occurs at or approximately at the same frequency as does the point of innection Pi of its effective impedance andthat the maximum value of the effective reactance is, at least approximately, half the maximum effective impedance of the-crystal (at abscissa'0.77) or XMFE (approx) An important application of these disclosures will be described later in this specification.

In Fig. 7o is shown a frequency sensitive element consisting of a simple crystal adapted to be used at B in Figs. 2, 3 and 3a, in each of which an element B without associated condenser trans- 1 mltter is shown. InFig. 7b is shown how the antiresonant frequency of a simple crystal may be nuctuated at will through the `intermediary of a condenser telephone transmitter C in series with the element; Its resonant frequency may also be fiuctuated at will through the intermedi-` ary` of the condenser telephone transmitter in sexies with the crystal as shown in Fig. 7c. These alternative combinations of a simple crystal and a' condenser telephone transmitter are adaptedA to be used at B in Figs. l, 4 and 3a, each of which shows an element B having a condenser transmitter associated. therewith. When the antiresonant frequency is fluctuated -by the transmitter C'. then the frequency of the constant frel quency source A isA adjusted to approximate one or the other of the frequencies corresponding to A the points of inflection P1 or Pz on the impedance frequency curve of the crystal at and near its antiresonant frequency. This impedance-frequency curve is curve l of Fig. 19.

When, on the other hand, the resonant frequency of the crystal is fluctuated by the series transmitter C" as shown in Fig.` 7c, then the constant frequency-source d is adjusted to approximate the frequency corresponding to the point of inflection on the impedance-frequency curve of the crystal at or near its resonant frequency `im. This impedance-frequency curve is the dotted curve I of Fig. 20. 'Ihe frequency of the source in this case may also be adjusted to correspond v to such a value as k=.l5, .20 or .25, and if the distortion due to the non-linearity of the impedance curve be more than is tolerable, then some of the known methods of diminishing or sensibly eliminating it can be used.

In Fig. 8a is-illustrated an improved frequency sensitive element in which the effective inductance reactance of the crystal (which as may be seen from curve 3 of Fig. 19 reaches its maximum value at about the point of inflection Pi of the impedance curve I) is lneutralized by the negative reactance of the condenser Ci. The reactance of condenser Ci is shown in the dotted curve 4 of Fig. 19. The dotted curve l shows the impedance of the combination oi' crystal K and condenser C1. This latter curve illustrates that tim combination of Fig. 8a. is markedly more frequency sensitive at or near the point of inflection P of its impedance curve than is the crystal K and capacityCr may be fiuctuated by. condensertransmitter C'. As in the case of Fig. 7b the modulator frequency should -be adiusted to the point of innection P where the combination is highly frequency sensitive.

Fig. 9a illustrates an improved frequency-sensitive combination in which an antiresonant element is secured byv placing two predeterminedly detuned crystals in parallel. tion the resonance frequency nn of K1 is so relatively detuned with respect to the resonance frequency nn of K: that at Aan intermediate frequency n. the effective reactance of the two crystalsshall be equal but opposite in sign. Under these circumstances n. becomes a pronounced antiresonant frequency. Forexample, referring to Fig. 19, the point of positive maximum reactance of crystal K1, if constructed to resonate at,the

proper frequency, might occur at a frequency corresponding approximately to the ordinate 0.758 (point P on curve I) and the maximum negative reactance might occurI at approximately the ordinate 0.784. Then if the crystal K: is constructed to have maximum negative reactance at 0.758, the reactances of the two crystals will oppose each other at this point. y, In Fig. 21 is shown the impedance frequ'ycy characteristic of such an organization when the selectivities of the crystals are S=4 105, their resistances are 38.1476 and the interval of detuning is l0'-5 ne cycles, A'I-'he delicate adjustment of the detuning of the two crystals of 'the organisation of Fig. 9a may best be effectedl by-means of an adjustable condenser connected in series with one of the crystals. Such a condenser is shown at C'" in Fig. 9b. Fig. 9b illustrates how the impedance of such any organization as that of Fig.l 9a may be caused to fluctuate in accordance with the same waves of speech or music through the association of a condenser telephone transmltter connected in parallel with crystal K as shown at C in Fig. 9b, oras shown in Fig. 9c the condenser microphone might be in series.

The theory of the improvements in frequency sensitive elements composed of combinations of crystals will be greatly facilitated by a consideration of the circuit equivalent of a' crystal, shown in Fig. 10, for frequencies in the neighborhood of its primary resonant and antiresonant frequencies and the mathematical expressions therefrom deduced for the behavior of the crystal throughout this range of frequencies.

From the circuit equivalent of the crystal as illustrated in Fig. 10, we deduce that the eil'ectlv or virtual resistance oi the crystal is that the virtual reactance of the crystal is and that therefore its virtual 7 0 In these expressions. R is the effective resistance of the branch CxLs of Fig. 10, S is the selectivity of the crystal or circuit equivalent, n is the impressed frequency, nr is the resonance frequency of the crystal which .is sensibly the same as that of the branch CyL of Fig. l0.- Be

In this combina- 4 1 ascisse cause of the high selectivity of the crystal we have A 1 -E*4JC,L. (4)

p is a constant to be determined and a is the frequency function oi' the branch C.-L. It is defined by the expression l nn=2 z (5) nf nf n C. is the real capacity between the electrodes and is therefore given by the expression ll, lK 10"n C= i; H9 farads (6) where K is the inductive capacity 'of quartz or 4.55, Zalm is the area of the faces of the crystal section and also the area of its electrodes. lolm= .84. le is the thickness of the crystal section. l=0.05.

This gives C=6f159 10-12 farads. 'l

The antiresonant point is sensibly reached when a :l a

P nu or by using the relationship o f Equation 2s &=.l 'nr nu P nu 0r i f Since these frequencies are readily determined we have an expression for vp which in the case of the samplar crystal of this specication gives For the antiresonance frequency, we have sensibly the expression Therefore substituting the value given by Equation 4 In the case of our samplar crystal this givesl Cf=4.172 -14 farads. We have L 412mm,-

which gives L=9.714 henrys for our sampler crysI tal for which resonance frequency nf=2.5000 105 and the antiresonant frequency n=2.5077 105.

By definition therefore From this formula we deduce that in the samplar whose selectivity is 104, the resistance R=1.526 ohms, while in the vacuum ground and polished crystal section whose selectivity is 4X105, the resistance R=38.148 ohms. It but remains to point out that the maximum value reached by the effective resistance R of the crystal at antiresonance is 5.7786 106 when S=104 and 23114)( 108 when S=4 105.

These formulae and dataytogether with the characteristic curves of the samplar crystals in Figs. 19 and 20 make the mode of functioningl of these crystals as frequency sensitive elements very clear and explicit. They enable one not only to apprehend their -mode of functioning but'also to have a quantitative understanding thereof. Thus we see that the crystal is the equivalent of an extremely selective resonant branch CIL shunted by a negative reactance, which, in the case of l oui samplar, is about -94,000 ohms inthe vicinity of the resonant and antiresonant points of the illustrated in Fig. 11 at La'.

crystal.

To, in effect, remove this capacity shunt from the crystal would imply an inductance shunt about the crystal of approximately 0.06 henry, as An vinductance of such a magnitude can, of course, not be supplied by a coil in the case of the high frequency currents contemplated in this specification, but in practice the attempt has been made to meet this diiculty -by shunting the crystal by a capacity C far greater than C. and then effectively neutralizing this far greater capacity shunt of C+C by a correspondingly smaller inductance. Such an organization is illustrated in Fig. 11a.

Thus if the added capacityC be 999C., then' the required inductance L will be approximately but 0.00006 henry. If the selectivity of the circuit CL comprising the added capacity and inductance be 200, the effective shunt on the crystal at its resonant point will be ofthe order of 5X 10lli ohms,`

while if great care be exercised in the construction of the added coil so that a selectivity of 400 is attained for the shunt loop, then the eiiective shunt on the crystal will be oi' the order of 2x10a ohms. 0f course theformula for tunin the inductance and capacity i8 A Since the inherent capacity reactance shunt is of the order of 10s it will be seen that no material advantage is secured by the device of Fig. lla

unless the coll is so carefully constructed as to secure the maximum available selectivity of 400 for the loop CL.

A better and simpler device for minimizing the effect of the capacity reactance shunt L 21rnrC., on the resonant branch CrL of the crystal is illustrated. in Fig. 12a. By-this device the capacity reactance shunt is neutralized bythe effective inductance of a second and detuned crystal K.V

The crystal K' is made resonant to a frequency 4nr' suilicientlybelow that of the crystal K to make its effective inductance reactance equal so that it forms an antiresonant circuit with the real capacity C of crystal K.. The reactance, resistance and impedance curves of. such a combination are shown in Fig. 22. If the selectivity of the adiunctcrystal K be 104, then the eifective impedance of the shunt to the branch CrL of crystal K will -be of the order of 10n ohms, while if the selectivity of the adjunct crystal be 4:)(10s then the effective impedance in question will -be of the order of 5x10. If the selectivity of the adjunct crystal K' be equal to or greater than that of the crystal K, it will in general be desirable to reduce it, and to this end a suitable resistance may be inserted at the points a or b.

The adjustment of the ratio of the'frequency of the adjunct crystal K' to that of the crystal K is too delicate to be made structurally, but this adjustment may be made to the desired degree of precision, by an adjustable condenser bridged between the points a and b, or by an adjustable condenser or inductance coil at a or b.

In Fig. 12b is illustrated the preferred manner in which the frequency of the organization of Fig. 12a may be caused to fluctuate in accordance with the vibrations of sound waves impinging on the diaphragm of the condenser telephone transmitter C'.

The frequency sensitive elements of the type of Figs. 12a and 12b are elements which are intended to be used not at. but near the resonance frequency nr. 'I'hey are intended for use as relatively low impedance elements. But Fig. 13a illustrates a high impedance or antiresonant frequency sensitive element in which .the combined reactance of the real or C. capacities of the crystals K1 and Ka is neutralized by the effective inductance L' of crystal K' at the antiresonant frequency of the element. This organization is therefore an antiresonant frequency sensitive element. A comparison of the frequency sensitive element of Fig. 13a with that of Fig. 9a reveals the fact that these organizations are the same except for the fact that in the case of the organization of Fig. 13a, the capacity shunt formed by the real capacities Csi and C.: of the crystals resonant frequency of the element is to change the negative reactance shunt of about 5 104 ohms to a high resistance shunt of about 1011 or 10 ohms at the antiresonant frequency nm of the element. As in the case of the frequency sensitive elements of Figs. 12a and 12b the adjustments of the rela.-

tive frequencies of the several crystals is too delicate a matter to be made to depend upon the structure of the crystal sectionsl alone. For this reason the nal adjustments are made by the shunt across between the points a and b. The serially connected adjustable condenser at a or b serves to adjust and fix precisely the frequency insertion oi' adjustable condensers in series at a or b and preferably also an adjustable condenser in interval of detuning between the resonant fre-v quencies ma and 'ni-1 of crystals Kn and K1, respectively, while the adjustable condenser in bridge across between the points a and bserves to adjust and fix precisely the capacity reactance to be neutralized by the inductance reactance of the crystal K'.

In Fig. 13b is illustrated a manner in which the frequency of the organizational. Fig. 13a may be caused to fluctuatel inaccordancewith the an effective capacity C' to a condenser considerably greater than its real capacity C. The condenser telephone transmitter C is shown in series with an inductance coil L. If C and L designate,

respectively. the capacity and inductance of these elements, then @ruim where wo is the periodicity of resonance of the combination. When this periodicity is greater than the impressed periodicity o, then the reactance of the -branch CL is a capacity or negative reactance given by the expression 2 w l: o

25. an indiscreet use of coil inductance will unduly the two branches ablcxd and abzczd of the bal-2 '-ance. The circuit I, l' is used to feed the necesvibrations of sound waves imping'ing pn the diaphragm oi' the telephone transmitter C'. lf de sired the transmitter may be connected in parallel as shown in C" at Fig. 13e.

In Fig. 14 is illustrated a means of imparting reduce the selectivity of the frequency sensitive element.

Fig. 15 illustrates a convenient method of adjusting the antiresonant frequency of a crystal. Just as an adjustable condenser immediately in series with a crystal is the most approved device for adjusting the resonance frequency nr of a crystal, an adjustable condenser immediately in parallel with a crystal is the most approved device for controlling and adjusting the antiresonant frequency ne of a crystal. This is because of the high Q which condensers may be given as compared with coils and because they are far more easily adjusted as to their capacity than are coils as to their inductance. In connection with the foregoing, it is pertinent to point out that the Q of a circuit, when measured at its resonant or antiresonant point is its selectivity S.

In Fig. 16, A is a source of constant high frequency E. M. F., being preferably a triode ampliiler, B1 and Bz are matched or balanced frequency sensitive elements controlled, respectively, as to their antiresonant freguency by the condenser telephone transmitters Ci and Cz. The branches alncid` and abzcnd are balanced, the equal loops Cala and Cda are tuned to the frequency of the source A. the equal loops CsLs and CLs are tuned to the frequency of the source. The secondary coil L1 is so symmetrically related to the two equal primary coils L: and Li that it receives only the energy of the carrier frequency of the current in the two branches abicid and abnczd of the bridge or balance which the organization of this diagram depicts. The secondary coll Ls is so symmetrically related to the two equal primary coilsl'ii adLe that it receives only the modulatedfcomponent ci' the current in sary amount of the energy of the carrier current back to the oscillator A to maintain it in conz stant and continuous oscillation at that -frequency.

The bridge is so` balanced that when C1 and C: are not acted upon byy soundV waves the branches I, 3 and I, I' are conjugate. When Ci and C: areequally but oppositely acted upon by sound waves, the impedances oi.'I branches abicid and abzczd are equally but oppositely influenced. The equality of this inuence may be secured,'to a degree adequate for most practical purposes, through the use of matched condenser telephone transmitters C1 end Cz as well as matched frequency sensitive elements B1 and B2.

A final and more refined adjustment of the bridge 5 may be made through the use of auxiliary reactances in one or both branches in accordance with well-known principles.

When the sound Waves act equally and in phase accuses opposite in sign to the reactance of said crystal at said second mentioned frequency, saidsecond element being so connected in the transmission upon C1 and C2, then the frequency elements 10 sensitive element are shifted with respecttoeach B1 and Bs should be of the type illustrated in Figs. 5, 7a, 9a or 13a, in which the impedance characteristic is substantially symmetrical abou*l the vertical axis through the antiresonant frequency. Then B1 and Ba are respectively de- 15 tuned so that B1 is antiresonant at frequency n(1+k1A) and B: at frequency 11G-1cm) where n is the impressed frequency. The frequency interval kiAn is so chosen as to bring theimpedv ance frequency curves of B1 and B2 to equal but 20 oppositely sloping inflection points at the impressed frequency n.

While this invention has been disclosed in certain particular arrangements merely for the purpose of` illustration, it is to be distinctly under- ,25

other in accordance with modulating waves.

4. A transmission circuit, a frequency sensitive element in said circuit including a crystal section antiresonant at 'a definite frequency and having a maximum effective reactance of one sign at a second frequency near but at one side of the frequency to which the crystal is antiresonant, said second frequency corresponding to the point of inflection of the effective impedance of the crystal, a second element in said circuit having a negative reactance equal but opposite in sign to the reactance of said crystal at said second mentioned frequency, said second element being so connected in the transmission circuit that a condition of substantially zero reactance exists in said circuit at said second frequency, and means for uctuating the antiresonant frequency of said crystal in accordance with sound waves.

5. A transmission circuit, a frequency sensitive element in said circuit including a crystal section antiresonant at a definite frequency and having a maximum effective reactance of one sign at a second frequency near but at one side of the frequency to which' the crystal is antiressecond frequency near but at one side of the onant, said second frequency corresponding to frequency to which said element is antiresonant, a second element in said circuit having a reactance substantially equal and opposite in sign vto the reactance of said antiresonant element at the point of inflection of the effective impedance of the crystal, a second crystal section connected in parallel with said'flrst section, said second Crystal section being so cut that its natural fresaid second mentioned frequency, said second ele- 40 CluenCy Will be detuned with respect to said rst ment being so connected in the transmission circuit that a condition of substantially zero reactance exists in said circuit at said second frequency, a high frequency Wave transmitted section by an amount which will cause its maximum effective reactance of sign opposite to that of said rst crystal section to occur at the same frequency as the maximum effective reactance by said frequency sensitive element, and means of said first section, whereby a condition of subwhereby the frequency of said high frequency wave and the second frequency of said frequency sensitive element are shifted with respect to each other in accordance with modulating waves.

stantially zero reactance will exist in said transmission circuit at said second frequency, a high frequency wave transmitted by said frequency sensitive element, and means wherebythe fre- 2. A transmission circuit, a frequency sensitive fluency of said high' frequency Wave and the secelement in said circuit, said element being antiresonant at a'definite frequency and having a maximum effective reactance of one sign at a second frequency near but at one side of the freond frequency of said frequency sensitive element are shifted with respect to each other in accordance with modulating Waves.

6. A transmission circuit, a, frequency sensiquency to which said element is antiresonant, a tive element in said circuit including a crystal second element in said circuit having a reactance substantially equal and opposite in sign to the reactance of said antiresonant element at said second mentioned frequency, said second element section antiresonant at a definite frequency andhaving a maximum effective reactance of one sign at a second frequency near but at one side of the frequency to which the crystal is antiresbeing so connected in the transmission circuit onant, said second frequency correSpODdiIlg t0 that a condition of substantially zero reactance exists in said circuit at said second frequency, :and

the point of inflection of the effective impedance of the crystal, a second crystal section connected in parallel with said first section, said second crystal section being so cut that its natural frequency will be` detuned with respect to said first section by an amount which will cause its maximum eective reactance of sign opposite to that of said first crystal section to occur at the same frequency as the maximum effective reactance sign at a. second frequency near but at one of said first section, whereby a condition of subside of the frequency to which the crystal is antiresonant, said second frequency corresponding `tothe point of inflection of the effective :impedance of the crystal, a second element in said stantially zero reactance will exist in. said transmission circuit at said second frequency, and means for fluctuating the antiresonant frequency of said rst mentioned crystal section in y circuit having a negative reactance equal but accordance with sound waves.

- quency, and each section having points of maximum positive and maximum negative effective reactances at frequencies near to but spaced from the antiresonant frequency of the crystal section/on either side of said resonant frequency, the tuning of said branches being such that their frequencies differ from each other by an amount whereby the maximum effective positive reactance of one crystal section and the maximum effective negative reactance of the other section will occur at the same frequency, said crystal sections being so cut that their antiresonant frequencies will differ from each other by an amount substantially equal to the detuning of said branches, a condenser associated with each of said crystal sections for aiding in precisely determining the detuning of the branches with respect to each other, a high frequency wave transmitted by said transmission circuit, and means whereby the frequency of said high frequency wave and the frequency at which substantially zero reactance is produced in said transmission circuit are shifted with respect to each other in accordance with modulating waves.

8. A transmission circuit having two parallel branches, each' branch having included therein a crystal section antiresonant at a denite frequency, and each section having points of maximum positive and maximum negative effective reactances at frequencies near tobut spaced from the antiresonant frequency of the crystal section on either side of said resonant frequency, the tuning of said branches being such that their frequencies differ from each other by an amount whereby the maximum effective positive reactance of one crystal section and the maximum effective negative reactance of the other section will occur at the same frequency, said crystal sections being so cut that their antiresonant frequencies will differ from each other by an amount substantially equal to the detuning of said branches, a. condenser associated with each of said crystal sections for aiding in precisely determining the detuning of the branches with respect toy each other, and means for fluctuating the capacity of one of said condensers in accordance with sound waves.

9. A transmission circuit, a frequency sensitive crystal in said circuit antiresonant at a de" nite frequency, means to practically annui the reactance of said crystal at a second frequency at one side of said antiresonant frequency and' corresponding to the frequency at which said crystal element has its greatest effective positive reactance, said crystal having such characteristics that it is equivalent in effect to a series resonant combination in parallel with its electrode capacity, means connected to said crystal to minimize its negative electrode capacity at frequencies in the neighborhood of the frequency at which it is antiresonant by opposing to said capaclty a positive reactance, a high frequency wave transmitted by said frequency sensitive crystal, and means whereby the frequency of said high' frequency wave and the second frequency of said crystal are shifted with respect to each .other in accordance with modulating waves.

10. A transmission circuit, a frequency sensitive crystal in said circuit antiresonant at a definite frequency, means to practically annui the reactance of said crystal ata second frequency at one side of said antiresonant frequency and corresponding to the frequency at which said crystal element has its greatest effective positive reactance, said crystal having such characteristics that it is equivalent in effect to a series resonant vcombination in parallel with its electrode capacity, a second crystal section' in parallel in said first crystal section, said second crystal section being detuned with respect to the first by such' an amount that its effective reactance at frequencies in the neighborhood of the antiresonant frequency of said first crystal section will be substantially equal to but opposite in sign to the electrode capacity of said first crystal section at frequencies in the neighborhood of its antiresonant frequency, a high frequency wave transmitted by said first mentioned frequency sensitive crystal, and means whereby the frequency of said high frequency wave and the second frequency of said first mentioned crystal are shifted with respect to each other in accordance with modulating Waves.

11. A transmission circuit, two crystal sections connected in parallel withl each other in said circuit and having antiresonant frequencies differing from each other by such an amount that the maximum effective positive reactance of one crystal at a third frequency on one side of its antiresonant frequency will be opposed by a substantially equal negative reactance of the other crystal at said third frequency, whereby the reactance of said transmission circuit will be substantially zero at said third frequency, each of said crystal sections being equivalent in effect to a series resonant combination in parallel with the electrode capacity of the crystal, means associated with said parallel crystals having such .electrical characteristics as to minimize the cffect of the combined negative electrode capacities of the two crystal sections at frequencies in the neighborhood of the antiresonant frequency of one of said crystal sections by opposing thereto a positive reactance, a high frequency wave transmitted by said transmission4 circuit, and means whereby the frequency of said high frequency wave and the third frequency at which substantially zero reactance is produced in said transmission circuit are shifted with respect to each other in accordance with modulating waves.

12. A transmission circuit, two crystal sections connected in parallel with each other in said circuit and having antiresonant frequencies differing from each other by such an amount that the maximum effective positive reactance of one crystal at a third frequency on one side of its antiresonant frequency will be opposed by a. substantially equal negative reactance of the other crystal at said third frequency, whereby the reactance of said transmission circuit will be substantially zero at said third frequency, each of said crystal sections being equivalent-in effect to a series resonant combination in parallel with the electrode capacity of the crystal, means associated with said parallel crystals having such electrical characteristics as to minimize the effect of the combined negative electrode capacities of the two crystal sections at frequencies in the neighborhood of the antiresonant frequency of one of said crystal sections by opposing thereto a positive reactance, and means to fluctuate the antiresonant frequency of one of said two crystal sections in accordance with sound waves.

13. A transmission circuit, two crystal sections connected in parallel with each other in said circuit and having antiresonant frequencies differing from each other by such an amount that the maximum effective positive reactance of one stantially equal negative reactance of the other crystal at said third frequency, whereby the reactance of said transmissioncircuit will be sub# stantially zero at said third frequency, each of said crystal sections being equivalent in effect to a series resonant combination in parallel with the electrode capacity of the crystal, a third crystal section connected in parallel with the first two, said third crystal section being detuned with respect to the rst two crystal sections so that its effective positive reactance at frequencies in the neighborhood of the antiresonant frequencies of said two crystal sections will minimize the combined negative electrode capacities of said sections at frequencies in the neighborhood of the antlresonant frequency of one of said sections, a. high frequency wave transmitted by said transmission circuit, and means whereby the frei quency of said high frequency wave and the third frequency at which substantially zero reactance is produced in said transmission circuit are shifted with respect to each other in accordance with modulating waves.

14. A transmission circuit, two crystal sections escasas connected in parallel with each other in said circuit and having antiresonant frequencies differing from each other by such an amount that the maximum effective positive reactance of one crystal at a third frequency on one side of its antiresonant frequency will be opposed by a substantially equal negative reactance of the other crystal at said third frequency, whereby the reactance of said transmission circuit will be substantially zero at said third frequency, each of said crystal sections being equivalent in veffect to a series resonant combination in parallel with -the electrode capacity of the crystal, a third crystal section connected in parallel with the rst two, said third crystal section being detuned with respect to the first two crystal sections so that its effective positive reactance at frequencies in the neighborhood of the antiresonant frequencies of said two crystal sections will minimize the combined negative electrode capacitiesof said sections at frequencies in the neighbor- -hood of the antiresonant frequency of one of said sections, and means to fluctuate the antitais in accordance with sound waves.

JOHN s'roNE STONE. 

